The invention relates to rotary electrical machines having a superconducting field winding.
Electrical machines adapted to operate as A.C. motors or generators have been described which comprise a superconducting field winding constructed to be supplied with direct current for producing a magnetic field having n pairs of poles (n being an integer), an armature having polyphase windings of normal electrical conductivity which deliver a field having n pairs of poles which rotate with respect to the armature when they are circulated by polyphase currents, and auxiliary windings of a material having normal electrical conductivity which deliver a field having n pairs of poles when D.C. currents circulate therein. Such machines may be of the synchronous type, as described in U.S. patent application Ser. No. 443,014, now abandoned. They can also be asynchronous, as described in U.S. Pat. No. 3,742,265 (Smith). The field winding, auxiliary windings and armature windings are located concentrically or coaxially in side by side relation.
In the above constructions, the field winding rotates in operation. As a consequence, sliding seals should be provided on the circuit for feeding the cooling fluid (generally liquid helium) to the cryostat of the field winding and returning it. The design of sliding seals adapted to operate satisfactorily at the cryogenic temperatures is a problem.
On the other hand, British patent specification No. 1,289,092 describes an A.C. generator having a stationary superconducting winding, an armature winding and supplementary coils. In operation, substantial axial forces are exerted on the superconducting field winding and the structure supporting the field winding should be so massive that it constitutes a heat flow path between parts at cryogenic temperatures and parts at ambient temperature which results in losses of such magnitude that a large cryogenic supply is necessary.